What Value is Filmic Using to calculate Maximum Scene sRGB Intensities?
If your question is which scene referred value would become the display referred (1.0, 1.0, 1.0) triplet after the Filmic View Transform has done its job,  the answer (currently) is:

(2 to the power of 6.5) * 0.18


which in numbers is

16.291740238....


The 0.18 is the scene referred value that Filmic deliberately has chosen as the 'middle grey' reference value. The 2 to the power of 6.5 part basically comes from the maximum value being defined as 6.5 stops over middle grey. A table of sample values is provided in the readme of Troy Sobotka's filmic-blender GitHub repository.

The value is taken from the resulting pixel values after the rendering stage. In photographic terms, that would be the amount of energy that lands on the film stock or digital sensor. So in essence, camera rays received.

Be aware that these values are deliberate choices by Troy Sobotka, and are bound to change with future iterations of Filmic. Also it's worth noting that any scene referred value above the given 16.291740238 will also give display referred (1.0, 1.0, 1.0) in sRGB color space.

User remco together with Troy himself pointed out to me:


  That 0.18 (or 18%) is the gray value that is (or was) considered to
  correspond to the gray that the human eye sees as halfway between
  black and white, and used on e.g. exposition targets (Kodak grey
  card). Thus, it's not a completely arbitrary choice.


It is important for me to point out that with deliberate i was never attempting to say arbitrary. Designing a View Transform like filmic-blender is both a matter of color science and craftsmanship. Thus there is some level of freedom for the one designing it to define the boundaries. The high clip value which the initial question was asking for is such a choice. It could technically be at a different energy level, but in a rendering context the choice made has proven to serve a high number of use cases well. The scene referred 0.18 middle grey value however is a convention which I missed in the initial answer, so thanks for the comments.